Page 7 - Vannucchi_Cappietti_2016

Sustainability 2016, 8, 1300 7 of 21

For each area, the points at water depths of 50 m and 15 m were selected and the wave roses
and the time series of the computed wave power values were extracted to describe the wave energy
potential within the studied domain. Furthermore, the hotspot was highlighted as the point where the
wave power is higher than the offshore value and higher than values in its neighborhood. These hotspot
areas have the highest potential as prospective energy farm sites, as defined by Iglesias [27] in 2010
and by Rusu [30] in 2012, providing that the so-called non-technical factors are also favorable [49].
For each hotspot, scatter diagrams and energy diagrams were developed in terms of significant
wave height and energetic period in order to visualize the occurrence of the different sea states and the
contribution of the different sea states to the total annual wave energy.

3.3. Performance Assessment of Wave Energy Converters
Among all proposed WECs that have been presented in the open literature, in the present work,
we have selected six (Table 1). More specifically, only those that were already tested in real sea state
(at least as a scaled model with scale factor larger than 1:5) and that already published their power
matrix as a measure of the actual performances [50–52] were considered. The selected WECs are:
(i) AquaBuoy [49]; (ii) AWS [53]; (iii) Pelamis [54]; (iv) Wave Dragon [55]; (v) Oyster [56]; and (vi) Wave
Star [57].

Table 1. Main features of the WECs considered in this study.

Position Type Power Take Off Rated Power (kW) Size
Offshore hydraulic motor/ diameter 6 m,
AcquaBuoy Point absorber 250
(>50 m) generator draught 30 m
Offshore 43 m deep
AWS Point absorber linear generator 2000
(>50 m) underwater
Offshore Attenuator— hydraulic motor/ diameter 3.5 m,
Pelamis 750
(>50 m) Oscillating Body generator length 150 m
Offshore Overtopping— width 300 m,
Wave Dragon water turbine 7000
(25–40 m) floating length 170 m
Nearshore Oscillating Body— width 18 m,
Oyster water turbine 800
(≈15 m) submerged height 12 m
hydraulic motor/ Float diameter
Wave Star Nearshore Multi point absorber 600
generator Ø5 m
The formula used to estimate the electricity production (P e ) of a WEC in a specific site is reported
in Equation (6) [42]:
nT nH
P e = ∑ ∑ P ij × f ij , (6)
i=1 j=1
where nT is the number of period classes; nH is the number of significant wave height classes, and,
for the i-th and j-th classes of period and wave height, is the power output of the device; and f is the
occurrence frequency obtained in the selected location.
The performance of a WEC was evaluated in terms of capacity factor (C ), defined in the
f
Equation (7) as the ratio between the total electrical power produced and the rated power [43] and in
terms of capture width (C w ), defined in the Equation (8) as the ratio between the electricity production
(P e ) of a WEC (in kW) and the period averaged flux of energy transported by the waves (P w ) per meter
of wave crest (kW/m) in each site [58].
P e [kW]
C = , (7)
f
Rated power [kW]
P e (kW)
c w = [m] (8)
P w (kW/m)